recognize the <a b c> form of alternation

Description

A <?before> rule with a space between the '<' and the '?' compiles
but fails at execution
with the unhelpful message :
Unable to find regex ''
Null PMC access in invoke()
--
cognominal stef

Followup:

I agree that this is not a useful error message. However, when
PGE is corrected to follow the specification the regex
< ?before foo>
would be identical to
'?before' | 'foo'
and no error message would be generated at all.
(PGE hasn't been updated to recognize the < a b c > form of
alternation.)
Pm